This invention relates to an improvement of the nitric acid separation process for the recovery of precious metals from silver refinery slimes which are obtained, for example, from the electrolysis of dore bullion produced from copper refinery slimes and, in paticular, to the purification of the gold sand resulting from the nitric acid process.
The recovery of precious metals has been practiced by the metals industry for many years and one important process is the recovery of gold and silver and other precious metals from electrolytic copper refinery slimes. Electrolytic copper refinery slimes contain a variety of elements including Ag, Au, Pt, Pd, Cu, Fe, Se, Te, Pb, Sb, Ir, Sn, As, Bi, Zn, and other elements. The recovery of precious metals from these complex slimes present a formidable problem for the industry.
In general, the slimes are first decopperized and then melted in a furnace with alkali fluxes to produce a dore bullion, which is cast into anodes and electrolyzed to provide silver crystals and anode slimes. The anode slimes, contain essentially all the gold and mosst of the platinum group metals, about 5 percent of the silver, and impurities such as copper, selenium, tellurium, lead and bismuth. A description of this process is in "Ab Outline of Metallurgical Practice", Third Edition, by C. R. Hayward on pages 108-112.
The anode slimes, commonly referred to as black gold mud, are treated to recover the gold and other precious metals and one method is by leaching with concentrated sulfuric acid to separate or part the silver from the gold as a silver sulfate solution. The resulting residue is commonly referred to as gold sand and precious metals such as palladium and platinum remain with the gold sand. These metals and other impurities interfere with the gold electrolysis step wherein a pure gold cathode product is separated from the cast gold sand anode. During the electrolysis, the precious metals and impurities build-up and contaminate the electrolyte which must then be replaced and processed to recover the precious metals. This procedure is costly and time consuming.
A nitric acid process has been developed as a more economic alternative to the sulfuric acid process and a detailed description of the process is provided in an article entitled "Recovery, by a Nitric Acid Cycle, of Gold and Platinum Metals from the Anode Slimes Arising from the Electrolysis of Dore Metal" by B. Tougarinoff, F. Van Goeltsenhoven and A. Dewulf, in Advances in Extractive Metallurgy, pages 741-758, The Institution of Mining and Metallurgy, 44 Portland Place, London, W.I., 1968, the disclosure of which is hereby incorporated by reference. Basically, the nitric acid process dissolves more of the silver and palladium than the sulfuric acid process and provides a gold sand which is purer and, thus, more efficiently electrolyzed.
Unfortunately, however, most of the platinum and some of the palladium and impurities still remain in the gold sand and during refining by, for example, the Wohlwill electrolytic process, gradually build-up and contaminate the electrolyte. In general, the electrolyte concentrations must be maintained below about 80 g. Pt/liter and 70 g. Pd/liter and when the electrolyte reaches these levels, electrolyte must be bled-off and replaced with new electrolyte. The bled-off (spent) electrolyte is then treated conventionally to separate the gold, platinum and palladium.
Gold sand may be conventionally purified before electrolysis by the well-known Miller process wherein chlorine gas is injected into the molten gold sand to convert various base metals into separable chlorides. TeCl.sub.4, for example, is volatilized, condensed and returned to the Dore furnace whereas silver is converted into a AgCl slag and is removed, reduced and returned to the electrolytic silver refining step. In general, the amounts of platinum group metals removed from the gold sand by this process are negligible and remain as contaminants in the gold sand.